Trubs, but no trianas: filled and empty regions of angiosperm stem length-diameter-mechanics space

Discrete plant habit categories such as ‘tree’, ‘shrub’, and ‘liana’ belie continuous variation in nature. To study the evolution of this continuous variation, we gathered data on stem length, diameter and tissue mechanical stiffness across a highly morphologically diverse highland xerophytic scrub on a lava flow in central Mexico. With stem allometric and mechanical data from 1216 segments from 50 species, we examined relationships between stem length–diameter proportions and tissue mechanical stiffness using linear mixed-effects models. Rather than a series of discrete clouds in stem length–diameter–tissue stiffness space, corresponding to traditional habit categories, the plants of this xerophytic scrub formed a single continuous one. Within this cloud, self-supporting plants had stems that became predictably longer and tissues that became stiffer for a given diameter increase, and there was no paucity of intermediates between trees and shrubs (‘trubs’). Non self-supporting plants had a steeper stem length–diameter slope and their tissues did not increase in stiffness with stem size. The area between self- and non self-supporting plants was sparsely occupied as stem size increased. We predict that this ‘empty’ space between lianas and trees is developmentally accessible but of low fitness, meaning that there should be few ‘trianas’ in nature.